"Missouri Severe Weather"
21 May 2000

On 21 May 2000, isolated severe thunderstorms broke out in northwest Missouri. They produced golf ball sized hail, up to 70 m.p.h. wind gusts and spawned a F1 tornado. Forecasts for this zone were for sunny skies through the afternoon, and the severe weather caught many by surprise. In hindsight, it was a subtle case, but conditions were present to produce the thunderstorms that afternoon.

On the evening of 20 May, there existed a broad upper-level trough across the eastern U.S. A shortwave in the flow was causing a line of showers and thunderstorms from Minnesota southwestward through southeast Colorado. By 1200Z 21 May, the shortwave had progressed east, the line of showers still intact. They were expected to move through eastern Missouri early in the day, with clearing and sunny skies for the afternoon. By mid-afternoon, skies had cleared across eastern Missouri, but a line of convection can be seen across eastern Iowa through northwest Missouri. By 2200Z, several isolated storms had grown across northeast Missouri and were producing severe weather.

There was not a cold front associated with the shortwave or the line of showers that moved through Missouri early in the day. The air ahead of the line of precipitation was cooler than the air behind it. As this boundary passed through Missouri, the skies cleared and warmer air was ushered into northeast Missouri on northwest winds. Aloft, due to the presence of the shortwave passing through the area, a pool of cooler air existed aloft across eastern Iowa and Missouri. This situation created an unstable atmosphere, with cooling aloft and warming at the surface. Also, there was a line of convergent flow at the surface, from the west to northwest wind in northeast Missouri and the southerly flow in central Missouri. This most likely gave the air the initial lift, and with the destabilization of the atmosphere over this area, led to the thunderstorm development.

Eta's 12Z 21 May run hinted at the chance for thunderstorm development with CAPE values in northwest Missouri from 700-900 J/Kg. ETA's 6-hour CAPE forecast showed a distinct axis of higher instability that coincided well with the initial line of storms that formed. The conditions the storms formed in weren't conducive to producing rotation and thus tornadoes, as the flow was from the west to northwest at all levels. But the storms were moving into an area that had southeasterly flow at the surface, veering with height. These conditions could produce mesocyclones; though like the forecast values for CAPE, were only borderline values for actually forecasting severe weather in advance, using the Eta. So as can be seen, forecasting severe thunderstorms from such subtle features would pose a great challenge to forecasters; while finding the features in hindsight is a much easier job.

Data Description

Data from this case cover the time period of 00 UTC on 20 May 2000 TO 06UTC on 22 May 2000 and include imagery from GOES-8 and the EAX WSR-88D radar in Pleasant Hill, MO (near Kansas City).

Case Study 33